Sibley has an excellent, neutral article about the ethics of using  
call playbacks to attract birds. I could paraphrase what he's written,  
but he's done a great job at addressing the pros and cons of using  
calls and how to do so responsibly, so I think it's definitely worth a  
read for all birders, whether or not you choose to use recordings.  
It's not a terribly long article, and highlights many key points that  
may help birders determine when may or may not be a good situation to  
use that new bird app on your Iphone, so check it out!

Here's the link to the article
http://www.sibleyguides.com/2011/04/the-proper-use-of-playback-in-birding/

Enjoy,
Tyler Pockette